With development of Long Term Evolution (LTE) network technologies, an LTE network has covered some urban areas and traffic hotspot areas. In a current communications network, the LTE network coexists with a 2nd Generation (2G)/a 3rd Generation (3G) network. Because the LTE network is in a construction phase, some LTE networks may not support a voice service, but some LTE networks may support a voice service. In the LTE network that supports the voice service, user equipment (UE) may initiate a voice service by using voice over LTE (VoLTE). VoLTE is a technical solution in which only an LTE network is used and a voice packet is transmitted by using an LTE packet switched (PS) network. Therefore, network elements in the LTE network need to work normally, so that the voice service can be normally used. In addition, when initiating the voice service, the user equipment in the LTE network may first fall back to a 2G/3G network that has a circuit switched (CS) domain, performs the voice service in the 2G/3G network that has the circuit switched domain, and then returns to the LTE network after a conversation is completed. The technology of switching from a packet switched domain of an LTE network to a circuit switched domain of a 2G/3G network is referred to as a CSFB technology.
In the prior art, when both CSFB and VoLTE exist in a network, there is a complete mechanism for selecting CSFB or VoLTE to make a voice conversation. For a calling scenario, a user equipment performs selection based on a capability of the user equipment and a network capability. For example, if the user equipment supports both VoLTE and CSFB, and a network side also supports both VoLTE and CSFB, the user equipment preferably selects VoLTE or CSFB according to a preference of the user equipment. If the user equipment supports only CSFB, the user equipment selects only CSFB even if a network side supports both VoLTE and CSFB. For a called scenario, a network side selects a path (called party domain selection) for a voice service. For example, when a user equipment has performed Internet Protocol multimedia subsystem (IMS) registration, and the network side supports VoLTE, the network side preferably selects VoLTE for the user equipment to perform a service. However, in this case, if a packet data network gateway (PGW) is faulty, a called party cannot be connected.